problems with homemade incubator

I built a homemade incubator from plans on byc. I took a cooler cut a hole in the top for glass to see inside two holes in the sides for ventilation a light bulb inside connected to a hot water thermostat along with a computer fan a plastic dish for water and a hydrometer/thermometer combo. Works great able to keep a pretty steady humidity % but the temp is another thing. The hydrometer/ thermometer tells me the high and low for the past few hrs. Well it started off going from 102 to 95 now I have it set so the high is 100 and low is 88. This is my first incubator and my first time incubating pretty sure from the people I've talked to and things I've read that wont hatch eggs. Any ideas on how I can get my temp more steady?

Yes the temp is pretty steady its in my house away from any air vents or windows. Where can I get a 500r thermostat? I have the vent holes covered with tape and cut holes and slits in it to help with the humidity and temp.

Best to have the BACK of the thermostat facing the light bulb and around two inches away. If you get it right, you can get almost zero temperature fluctuation. I have two links in my sig; the second one is to a page that explains it very well, especially the second one http://cmfarm.us/WHTincubator.html , second video on the page.

Installing the thermostat so that I cannot work well is universally the reason "everyone" thinks you get large temperature fluctuations with hot water heater thermostats. Follow this guy's instructions and you'll be very pleased with your home built incubator. I sure was! I get almost a full degree of temp fluctuation but I'll accept that!